The invention relates to tires and to treads for tires of vehicles intended to bear heavy loads, and in particular the tread patterns of such treads which are provided with incisions.
In order to improve the road-holding of a vehicle when travelling in particular on wet or icy roads, it is necessary that the treads of the tires on this vehicle have sufficient grip on the road surface. To this end, it is known to provide the tread of each tire with motifs in relief defined by cutouts (grooves and/or incisions), these cutouts having, inter alia, the function of acting as evacuation channels. Each motif may furthermore comprise at least one incision in order to create a larger number of ridges intended to break up the layer of water covering the roadway. xe2x80x9cCutoutxe2x80x9d is to be understood to mean a slit which may have a rectilinear or other type of trace, for example curved, undulating, whatever the section plane (longitudinal, transverse or oblique) in question.
Of course, a necessary compromise has to be achieved between the number of cutouts and the rigidity of the tread, because an excessive number of cutouts (grooves and/or incisions) results in a significant reduction in the rigidity of the motifs in relief. In particular, under the action of forces tangential to the contact surface of the tire with the roadway, in particular on cornering, the motifs may be excessively deformed, which then results in a loss of road-holding capability.
In order to solve this problem, it was proposed to make incisions, in which the opposing walls are provided with means arranged to restrict the relative movements of one wall in relation to the facing wall.
For example, in FR 2 025 124 it is proposed to make incisions, the surfaces of the main opposing walls of which defining said incisions have a relief while remaining substantially parallel to each other, whatever the trace obtained on any section plane.
Furthermore, U.S. Pat. No. 5,783,002 describes a tread which makes it possible to obtain optimum properties of the elements in relief of this tread throughout the useful life of the tire equipped with the tread. This tread comprises at least one incision defined by two facing walls, each having a surface in relief formed of protrusions and cavities which are arranged in a specific manner on either side of an average wall surface, said protrusions of a wall being intended to cooperate, by interleaving, with the cavities of the facing wall, and vice versa.
The solutions described in these documents have proved to be of very high performance for tires intended for passenger vehicles or more generally vehicles bearing relatively light loads. However, it was noted that when these same solutions were applied to the case of tires for vehicles bearing heavy loadsxe2x80x94in particular heavy vehiclesxe2x80x94the loads borne by the different tires created high contact pressures and hence high loading pressures of the motifs in relief of the treads. Under these contact pressures, the rubber mix constituting each of the motifs in relief of the tread is squashed and tends, by what is called the Poisson effect, to be deformed in all the transverse free directions relative to the direction of the contact pressures, that is to say in the directions in which said mixture is not materially limited. Under this effect which results from the compression, it is observed that the protrusions and cavities with which a first main wall is provided, which defines an incision with a second main wall facing it, cannot cooperate with all the protrusions and cavities of said second wall, each of said main walls being deformed during the loading of the tread in non-homogenous (non-regular) manner. In fact, it is noted that only certain parts of the walls of the incision come into contact and ensure the blocking function of said walls against each other (these contacting parts correspond to those parts in which the deformation by Poisson effect is at a maximum). It is furthermore noted that the deformation by Poisson effect is modified in the vicinity of the points of contact with the roadway according to the contact conditions (sliding possible or not possible between the contact surface of the tread and the roadway).
One object of the present invention is to form a tread for a tire which is intended to bear high loads and comprises incisions which do not have the disadvantages which have been discussed above.
To this end, the tread according to the invention manufactured in a rubber-type material comprises at least one incision which is defined by opposing walls (that is to say, ones which face each other), each wall being defined geometrically by an average surface bearing elements in relief, the elements in relief (protrusions, cavities) of a wall being intended to cooperate with the elements in relief of the facing wall in the contact of the tread with the roadway.
In order to obtain more effective cooperation of the walls of an incision which is defined by two walls provided with elements in relief (hollows and protrusions), the width of the incisionxe2x80x94that is to say the distance between the walls 3, 4 measured parallel to the running surface 11 when newxe2x80x94is not constant in all directions, and is variable at least in the direction of the depth of the incision. More specifically, the width of the incision according to the invention is at a maximum in that region of the tread which undergoes the maximum deformation due to the Poisson effect resulting from the loading of the tread in the contact with the roadway.
In this manner, the cooperation of the opposing walls of an incision is more effective since all the motifs in relief on one wall cooperate with all the motifs in relief of the facing wall, the difference in incision width being compensated by the transverse deformation of the rubber mix upon the loading of the tread.
In a first simple variant embodiment, the variations in the width of the incision according to the invention are substantially identical whatever the section plane perpendicular to the running surface of the tread and substantially perpendicular to the average surface of said incision (defined as the surface passing halfway between the opposing walls defining the incision).
Preferably, the maximum width of the incision is located substantially at a depth greater than or equal to half the depth of said incision.
In another variant embodiment, the curves of iso-width of the incision are curvilinear and concentric about a central region of the motif of the incision, said central region being located substantially at half the depth of the incision and at equal distance from the lateral ridges formed by said incision. xe2x80x9cCentral region of the motif of the incisionxe2x80x9d is understood to mean the region which is substantially at equal distance from the running surface and from the base of the incision and at equal distance from the lateral ends of said incision.
In practice, if a wall surface of an element of rubber mix defining an incision according to the invention is displaced to flatten it against the facing wall surface defining the same incision, there is obtained:
outside the zone of contact of the tread with the roadway, partial contact, that is to say contact only on the parts limiting said surfaces (these parts comprise the ridges formed by the incisions on said walls);
in the zone of contact of the tread with the roadway, total or virtually total contact of the surfaces of walls which are deformed in the transverse directions by the Poisson effect, this deformation locally compensating for the differences in initial width of the incision.
It is thus possible substantially to improve the blocking of the opposing walls of the incision upon loading. This is particularly advantageous when the tread is new, since after partial wear of said tread the incisions have decreasing depths and consequently the need to block the walls defining said incisions also decreases.
The tread pattern according to the invention may be applied to a tire of any type, it being understood that the distribution of the widths of an incision is a function of the transverse deformations due to the Poisson effect during the loading of the rubber blocks of the tread in contact with the roadway.
An incision according to the invention can be produced easily by conventional means such as, for example, using a blade having suitable surface geometries and distribution of thicknesses.
The characteristics of the invention will become apparent from the following description, with reference to the attached drawings and showing, by way of non-limitative examples, embodiments of the invention.